As for the use in general industry, for example, a driving motor apparatus for air conditioning apparatuses (hereinafter referred to as an “air conditioner”) is known. With the progress of air conditioners for higher performance and greater multifunction in recent years, the air conditioning is performed under the control of, for example, an inverter such that after the rapid cooling by a high-speed operation to lower the room temperature within a short time, the temperature of the room is kept constant by a low-speed operation. Accompanying this, a low-noise operation is demanded at the time of low-speed operation while suppressing the air blowing out sound, motor rotating sound and the like. However, the cooling efficiency inside the apparatus decreases at the low-speed operation and the temperature of rolling bearing installed into the motor sometimes rises even to around 100 to 120° C. Furthermore, the oil film thickness to be ensured by the lubrication can be hardly attained and the grease packed in the bearing is readily deteriorated. If the deterioration proceeds, noise is generated.
The outdoor unit used for air conditioners causes a bearing initial sound (cage sound) at the starting of operation in a low-temperature environment such as winter season and this becomes a problem in some cases.
Furthermore, a motor apparatus used for air conditioners is sometimes transported a long distance from a manufacturer of the motor to an end user by a truck while laying a rolling bearing in the state of being installed into the motor apparatus. In such transportation, the truck reflects fine irregularities on the road and this is transmitted to the rolling bearing as an impact load repeatedly added. As a result, the rolling element of the rolling bearing is repeatedly put into microscopic contact with the raceway surface to cause fretting damage (abrasion) on the raceway surface and this sometimes generates noises.
On the other hand, by taking account of the environmental regulation at the same time with the above-described higher performance and greater multifunction, studies are being made to accelerate the downsizing and the low outputting and thereby suppress the heat generation from motors. To satisfy this, the rolling bearing for these uses is demanded to have low torque properties as an important function. The dynamical friction torque of the rolling bearing is caused by the friction resulting from microscopic sliding on the rolling contact surface, the sliding friction at the sliding contact part within the bearing, and the viscous resistance of grease. Among these causes, the viscous resistance of grease is known to be under the effect of the kinematic viscosity of base oil and the penetration of grease. The kinematic viscosity of base oil is dependent on the oil shearing resistance when a fluid lubricating film is formed. Therefore, the reduction in this kinematic viscosity takes an important role in attaining the reduction in dynamical friction torque of the rolling bearing. The penetration of grease is related with the channeling property when the grease in the bearing is subjected to shearing at the rotation of the bearing. Therefore, the reduction of this penetration is also effective.
However, the motor of air conditioners is sometimes operated at a relatively low speed under the control of inverter as described above, therefore, if the kinematic viscosity of base oil is reduced, the oil film thickness can be hardly ensured. In addition, oils having low kinematic viscosity are generally low in the heat resistance and therefore, suffers from a problem in the acoustic durability. On the other hand, since the reduction of grease penetration incurs increase in the amount of thickener blended, the amount of base oil in the grease is relatively reduced and the resistance of grease against mechanical shearing is elevated, as a result, the amount of base oil supplied to the bearing lubricating surface is decreased and the lubricity cannot be stably maintained for a long period of time.
As such, reduction in the kinematic viscosity of base oil and reduction in penetration of grease are limited and in the case of a rolling bearing for the above-described uses, a grease such that the kinematic viscosity of base oil at 40° C. is from 10 to 500 mm2/s, the penetration of grease is NLGI No. 2 to No. 3 grade or the amount of thickener blended is from 5 to 20% by mass, is considered suitable. For motors required to satisfy particularly the low noise property, namely, the acoustic durability, a grease comprising an ester base oil having blended therewith a fatty acid lithium salt as a thickener is generally used. This is because the ester oil has high heat resistance as compared with mineral oils and a polar group which is contained in the molecular structure of the ester oil exhibits an activity of elevating the adsorption to the metal surface, whereby good abrasion property is ensured and the acoustic durability is improved. In the case where the reduction of fretting damage (abrasion) is required, use of a base oil having a relatively high viscosity and thereby exhibiting high oil film forming property is considered effective.
The saving of electric power and high speed rotation are being promoted also in the linear acting apparatuses such as linear guide apparatus and ball screw apparatus and a technique for attaining lower torque is considered important. Particularly, in the linear guide apparatus or ball screw apparatus required to satisfy high speed rotation and high torque output, the abrasion or torque increases accompanying the rising of output torque or rotational speed and this causes heat generation or motor load which obstructs the electric power saving and high speed rotation. Furthermore, the abrasion accompanying the rising of rotational speed gives rise to ball flaking.
Similarly to the above-described dynamical friction torque of rolling bearing, the torque of linear acting apparatuses is generated by the abrasion resulting from microscopic sliding on the rolling contact surface, the abrasion at the sliding contact part, the viscous resistance of grease or the like. Among these, the viscous resistance of grease is dependent on the kinematic viscosity of base oil in the grease and on the penetration of grease. Therefore, reduction in the kinematic viscosity of base oil and the increase in penetration are effective for the reduction of torque. However, if the kinematic viscosity of base oil is lowered, the thickness of oil film on the lubricating surface decreases and this sometimes adversely affects the lubricating performance. On the other hand, if the penetration is elevated, the content of the thickener in the grease relatively decreases and the resistance against mechanical shearing is diminished, as a result, the grease may be softened and leaked during operation or there arises a problem in the impact resistance.
From these reasons, a grease where the kinematic viscosity of base oil at 40° C. is from 10 to 500 mm2/s and the penetration is NLGI No. 2 to No. 3 grade, is conventionally packed in linear acting apparatuses.
Also, the torque may be effectively lowered by the reduction in the amount of grease packed, which allows, however, easy occurrence of abrasion or ball flaking due to lubrication failure.
As described above, higher performance and greater multifunction of air conditioners are required and as for the specification of ball bearing installed therein, more improvement in the acoustic property, reduction in the fretting damage (abrasion) and lower torque are demanded by taking account of the environmental regulation. These requirements increase highly probably in future. The same applies to linear acting apparatuses such as linear guide apparatus and ball screw apparatus.